It is presently known in the worldwide industry the need to produce biodegradable and biocompatible materials by using renewable raw materials and energy sources through processes that are not aggressive to the environment.
In modern society, the use of plastic materials in large scale is leading to a diversity of increasingly serious environmental problems. In view of these problems, the development of biodegradable plastic resins has been receiving worldwide attention, mainly those produced by means of a clean technology using renewable sources. The applications of these biodegradable biopolymers with best chances of being successful in the market involve products, such as disposable materials, for example packages, cosmetic and toxic agrochemical recipients, medical and pharmaceutical articles, as described in the introduction of Brazilian patent application PI04005622-1 (PCT/BR04/000237) of the same applicant. An important family of the biodegradable biopolymers is the polyhydroxyalkanoates (PHAs), which are polyesters naturally synthesized by a large number of live beings. With more than 170 representatives described in the literature, the commercial interest in PHAs is directly related not only to the biodegradability but also to their thermo-mechanical properties and production costs. Thus, only some PHAs have found industrial application, the most representatives being PHB (poly-3-hydroxybutyrate), PHB-V (poly(hydroxybutyrate-co-hydroxyvalerate)), P4HB (poly(4-hydroxybutyrate)), P3HB4HB (poly(3-hydroxybutyrate-co-4-hydroxybutyrate)) and some PHAmcl (polyhydroxyalkanoates of medium chain), the typical representative of this last family being PHHX (polyhydroxyhexanoate).
The chemical structure of the PHAs may be described as a polymeric chain formed by repetitions of the following unit:
Where R is an alkyl or alkenyl group of variable length and m and n are integers, in the polymers mentioned above R and m assuming the following values:    PHB: R═CH3, m=1    PBH-V: R═CH3 or CH3—CH2, m=1    P4HB: R═H, m=2    P3HB-4HB: R═H or CH3, m=1 or 2    PHHX: R═CH3—CH2—CH2—, m=1
The great development of the natural sciences in the last two decades, particularly in biotechnology, has allowed the use of the most different natural or genetically modified organisms in the commercial production of PHAs. Particularly relevant for the present invention is the use of determined bacterial strains, which are able to produce and to accumulate expressive quantities of these polymers in their interior. Cultivated in specific conditions, which allow reaching high cellular density, high content of intracellular polymer and yields compatible with the industrial process, these bacterial strains can use different renewable raw materials, such as sugarcane juice, molasses or hydrolyzed cellulose extracts.
Although attempts have been made for applying the bacterial cells in natura (without using PHA solubilizing agents), as moldable material, such as disclosed in U.S. Pat. No. 3,107,172, the commercial applications of PHAs in most cases require its purity to be sufficiently high to attain the desired plastic properties. In order to achieve the appropriate level of purity for processing the biopolymer, especially PHAs, there are normally required steps in which the utilization of solvents for extraction and recovery of the PHA from the residual biomass is obligatory.
In patent EPA-01455233 A2, there are described several possibilities to carry out the digestion of an aqueous suspension of cells containing PHA, by using enzymes and/or surfactants to solubilize the non-PHA cellular material, considering that the enzymes are very expensive and cannot be recovered in the process, unlike what occurs when the solvent is used. Also, high dilution of the cellular material is required, which leads to a great volume of effluents generated in the process.
The usually proposed extraction processes basically consist in exposing the dry or humid cellular biomass containing the biopolymer to a vigorous contact with a solvent that solubilizes it, followed by a step in which the cellular residue (debris) is separated. The solution containing the biopolymer then receives the addition of an insolubilizing agent, which induces its precipitation in the solvent (see, for example, Brazilian patent PI 9103116-8 filed on Jul. 16, 1991 and published on Feb. 24, 1993).
In the extraction processes through organic solvents, the solvents utilized are partially halogenated hydrocarbons, such as chloroform (U.S. Pat. No. 3,275,610), methylene-ethanol chloride (U.S. Pat. No. 3,044,942), chloroethanes and chloropropanes with boiling point within the range from 65 to 170° C., 1,2,3-dichloroethane and 1,2,3-trichloropropane (patents EP-0014490 B 1 and EP 2446859).
Other halogenated compounds, such as dichloromethane, dichloroethane and dichloropropane are cited in U.S. Pat. No. 4,562,245 (1985), U.S. Pat. No. 4,310,684 (1982), U.S. Pat. No. 4,705,604 (1987) and in European patent 036,699 (1981) and German patent 239,609 (1986).
The processes of extraction and purification of biopolymers from biomass which utilize halogenated solvents are totally prohibitive nowadays, since they are highly aggressive to the environment and to human health. Therefore, a solvent to be used as a potential extractor of the biopolymer from a cellular biomass should first fulfill the condition of not being aggressive to the environment.
In this sense, Brazilian patent PI 9302312-0 (filed on 1993 and granted on Apr. 30, 2002) presents a process of extracting biopolymer from bacterial biomass which employs as solvents high chain alcohols with 3 carbons or the acetates derived therefrom. This patent prefers isoamyl alcohol (3-methyl-1-butanol), amyl acetate (or amyl-acetic ester) and fusel oil, a mixture of high alcohols obtained as a by-product of the alcoholic fermentation and which has as main component the isoamyl alcohol. This patent is also characterized for using a single solvent as extractor and purifier, not requiring the utilization of an insolubilizing agent or counter-solvent and/or marginal non-solvent. The precipitation of the solute (biopolymer) of the PHA solution is carried out by cooling the solution.
U.S. Pat. No. 6,043,063 (filed on Apr. 14, 1998 and granted on Mar. 28, 2000), U.S. Pat. No. 6,087,471 (filed on Apr. 14, 1998 and granted on Jun. 11, 2000) and the international patent application WO-98/46783 (filed on Apr. 15, 1997) mention an extensive list of non-halogenated solvents which could be potentially employed as solvents for extracting biopolymer from biomass, but many of them presenting characteristics such as difficult industrial manipulation, toxicity, besides high cost. In said extensive list, which also includes the solvents cited in Brazilian patent PI 9302312-0, only a small number of solvents have potential to be industrially used for extracting biopolymer from bacterial or vegetal biomass either due to problems regarding incompatibility with the biopolymer, or due to their toxicity, explosiveness, and high cost.
Since the biopolymers are heat sensitive, that is when submitted to temperatures above a determined value, they degrade irreversibly, losing molecular weight, which can definitively affect the properties that characterize them as thermoplastics, it is fundamental to have in mind that the list of solvents with potential to be industrially used becomes even more restricted. The potential for industrial utilization of the solvent elected to promote the extraction of the biopolymer will be increased if it is associated with an appropriate process that allows extracting the biopolymer without causing significant alterations in its molecular weight. Although various applications use PHA with low molecular weight in the range from 10,000 to 50,000, it is desirable molecular weights superior to 500,000 Da in a much greater range of commercial applications.
In order to produce polyhydroxyalkanoates (PHAs) with molecular weight as close as possible to the original molecular weight, i.e. of the PHA when inside the cell, it is important to consider the case in which the PHA solvents need to be heated above 70° C. to solubilize the biopolymer; the longer it remains exposed to this temperature during the processing, the more it will degrade, which fact can irremediably impair its thermoplastic properties, as discussed in the previous patent application mentioned above PI04005622-1 (PCT/BR04/000237), of the same applicant.
For example, the polyhydroxybutyrate, originally containing a molecular weight of 1,000,000 Da and submitted to an extraction in isoamyl alcohol at 110° C. would yield, for a time of 5 minutes of exposure, 951,230 Da; for 15 minutes of exposure, 853,692 Da; for 30 minutes of exposure, 707,410 Da; for 60 minutes, 414,771 Da; and for 90 minutes, 122,230 Da.
Considering that, besides the extraction, other operations, such as evaporation and drying of the solvent are necessary to obtain a pure product with good mechanical properties, and that these operations many times expose the biopolymer to critical situations regarding the material, it is not difficult to imagine the inherent difficulties of processing this type of material. Besides the solvent, it is desirable to have an appropriate process that does not degrade the product thermally.
Bearing in mind the biopolymer heat sensitiveness and for purposes of producing material with high molecular weight, it should be taken into account that the potential for industrial utilization of the solvents, for example the solvents mentioned in U.S. Pat. No. 6,043,063, is intimately related to the time of exposure of the biopolymer during the process of extraction and recovery to those high temperatures which will define the thermal degradation level suffered by the biopolymer. In the cited patents, there is no reference to the obtained material properties, especially that related to the product molecular weight.
Other relevant fact regarding the industrial viability of this mode of PHA extraction is that, since it is a process of intensive energy consumption, the viability of the product is also intimately related to the availability of a low cost renewable source of energy. Considering the facts exposed above, it is possible to conclude that the industrial processes for producing PHAs should contemplate: strains of microorganisms presenting high efficiency in converting raw material into polymer, with a simple and efficient production protocol; raw materials of low cost and high yield; a procedure for extracting and purifying the polymer which allows obtaining a product of high purity, preserving at maximum the original characteristics of the biopolymer, with high yield and efficiency and through processes that are not aggressive to the environment.
Besides these economical aspects, since it is a more environmental friendly product, the whole process thereof should be compatible. Thus, the use of environmental harmful products in any production step should be avoided. Moreover, the source of energy used to run the process of production should come from a renewable source. It would not make sense to produce a plastic of low environmental impact if, for example, only non-renewable sources of energy are employed.
A renewable and cheap energy allied with the availability of cheap raw materials—sugar and molasses—and natural solvents obtained as by-products of the alcoholic fermentation makes the sugar and alcohol industry the ideal cradle for the production of bioplastic.
Aiming to efficiently obtain a PHA with high purity and high molecular weight from raw material of low cost, the previous patent application PI 04005622-1 (PCT/BR2004/000237) of the same applicant proposes a process for recovering PHAs from a bacterial cellular biomass, which is obtained by fermentation, said process presenting characteristics of non-aggression to the environment, by employing non-halogenated solvents.
Considering the suitability of the solvents for carrying out the process, the latter uses, as raw material, a cellular biomass slurry in aqueous suspension and with a dry cellular material content not inferior to about 18 by weight, which slurry is submitted to a solvent, agitation and heating in a reactor, in order to produce a suspension comprising solvent and dissolved PHA, which is subsequently separated, still hot, from the insoluble residues of the remaining cellular biomass. Then, the PHA and solvent solution is cooled to a temperature that allows the precipitation of the PHA and is submitted to a micro-filtration, in order to obtain a PHA paste with a PHA concentration of about 5% or more, which paste is heated and agitated again by injection of water vapor, in order to eliminate the solvent and to form an aqueous suspension of PHA granules, which will be submitted to a final separation.
Making use of the suggested solvents in the previous process described above, the concentration of dissolved PHA in the solvent is inferior to 5%, requiring a micro-filtration operation in order to allow said PHA concentration in the suspension to reach a minimum value, allowing the continuity of the process in terms of economical viability. In order to preserve the biopolymer molecular weight, the solution is rapidly cooled before being micro-filtrated.
Thus, in the previous process the requirement of energy is relatively large, because the large volume of diluted suspension originally heated is cooled and then re-heated before the solvent evaporation.
Moreover, this process requires facilities provided with high cost equipments necessary to carry out the rapid cooling and the micro-filtration steps.
Considering the low PHA solubility in the mentioned solvents, the volume of the circulating solvents required in this process is high, requiring pumping plant and larger equipment, besides higher quantities of electric and thermal energy for operation thereof and to solvent evaporation.